The invention refers to a printing assembly.
When printing on webs of paper, plastics foil or the like, an ink is transferred from an inking unit including an ink trough and/or a wiper, onto the web running along a counter-pressure cylinder. Generally, the printing of the continuously moving material web is done according to the flexo-printing or the gravure printing techniques. In such printing techniques, it is possible to employ inks that are curable by ultraviolet radiation or by electron beam treatment. It is an advantage of such inks that they allow for a fast curing without expelling auxiliary carrier agents, such as solvents of the inks applied onto the foil web. Alternatively, one may also use solvent-containing inks that generally present the disadvantage of solvent evaporating into the environment and color changes occurring after the application thereof.
Printing inks, in particular such printing inks as are curable by ultraviolet radiation or electron beams, do not change in consistency, for example, by loss of solvent, but their viscosity strongly depends on temperature. Printing assemblies processing such inks require a very precise temperature setting so that the inking is effected in the respective desired amount of ink to be applied. A change in the temperature of the liquid ink is only possible over the entire width of the roller. It may occur that the thickness of the layer of ink varies between different portions of the web length due to different gap widths of two cooperating rollers or between a roller and a wiper. Moreover, it often happens that ink is to be applied in layers of different thickness in different portions of the width of the material web. Different layer thicknesses may cause different hues in the respective portions, even if the color is the same. In some cases it may even be desired to create different hues at different portions of the material web, while still using the same ink, by applying the ink in layers of different thicknesses. The current systems for controlling the temperature, employed when using inks having a highly temperature-dependent viscosity, have their effects on the entire width of the material web or the entire width of the counter-pressure cylinder or the inking system.